Semiconductor lasers capable of producing continuous stimulated radiation at wavelengths in the vicinity of 1.1-1.7 um (micrometers) at room temperature are of interest for communications systems using fiber optics, since it is in this wavelength range that both the transmission losses and the dispersion in high-quality glass fibers are low.
Semiconductor lasers of quaternary III-V alloys of GaInAsP grown on a binary compound of InP (double-heterostructures or DH) have proven practical for operation at this frequency range. Furthermore, a particular type of laser construction, i.e., the buried layer type or BH laser for "buried heterostructure" laser; wherein the active layer (GaInAsP) is both vertically and laterally confined (Proceedings of the IEEE, Vol. 64, No. 10, Oct. 76, pp 1528-1529) has been of particular importance for reducing threshold current I.sub.th, and for increasing mode stability in semiconductor lasers.
As a result of this interest, various techniques for fabricating BH laser diodes have evolved.
Recently, a novel technique for fabricating BH lasers was described in U.S. Pat. No. 4,468,850 to Liau et al. issued Sept. 4, 1984, which utilizes a "mass transport phenomenon" to bury the active layer. In this device, an active layer of quaternary III-V alloy is grown on a binary III-V compound substrate using conventional liquid phase epitaxial (LPE) techniques. A top layer of a binary III-V compound is similarly epitaxially grown on the active layer.
Next, an oxide stripe mask is formed by conventional photolithography techniques on the top layer.
An undercut mesa structure is then formed by a two-step selective chemical etch. A first etchant is used to remove the top layer where it is not protected by the oxide coating. This top layer is removed down to the active quaternary layer at which point the first etchant step is immediately terminated and a new etchant is used to remove the active layer underneath the remaining top layer, except for a thin volume of active material underlying the remaining top mesa structure.
Next, the structure formed as above, is subjected to a controlled temperature cycle which produces a transport of material so as to fill in the void left at the undercut region and thereby enclose the sides of the remaining volume of the active material.
Lastly, ohmic contacts are provided across the device to enable current to be passed through the structure to produce lasing.
This process produces an improved double heterostructure laser diode of the BH type wherein the active layer is uniformly and narrowly defined, conventional epitaxial regrowth on a non-planar surface is eliminated, the yield is high and the threshold current is low.
Another application of the "mass transport phenomenon" is described in U.S. Pat. No. 4,468,850. In this application, the "mass transport phenomenon" is used to improve fabrication of etched mirror surfaces of laser diodes. This process is shown in FIG. 1 herein, labelled Prior Art. FIG. 1 is a progression of sectional side views of a buried heterostructure laser diode in which a mirror surface is formed on the surface of the diode shown by the arrow M in FIG. 1a. The starting structure consists of a n-doped indium phosphide buffer layer 14 formed over a n-doped indium phosphide substrate 16. A very thin gallium indium arsenide phosphide active layer 18 is formed over layer 14 and a p-doped indium phosphide top layer 12 is formed over the active layer 18.
Next, an oxide coating 20 is applied to the top layer 12 to form a mask by conventional photolithography techniques which ends at surface 42.
After the sample is fabricated, as described above, it is placed in an etchant bath of HCL for a sufficient length of time, such that the indium phosphide material 12, that is not under the oxide 20, is etched away. The etchant process is continued for a little while longer until the indium phosphide is etched a small distance (A), providing an overhang 43 underneath the oxide 20 (See FIG. 1b).
In the next step of the process, (step 3 shown on FIG. 1c), the oxide 20 is removed by conventional techniques and the gallium indium arsenide phosphide quaternary active layer 18 is etched away using an aqueous solution of KOH and K.sub.3 Fe(CN).sub.6 (potassium ferricyanide). This etching process is allowed to continue until an undercut region is produced for a distance (B), (as shown in FIG. 1c). Next, the sample is subjected to a heat treatment, in which it is heated to a temperature of about 670.degree. C., in an LPE chamber with flowing phosphine gas.
This temperature treatment results in transport of the indium phosphide in layers 12 and 14 to fill in the undercut 44 of FIG. 1c, resulting in the structure shown in FIG. 1d.
Next, as shown in FIG. 1e, an oxide 20' is formed on the top layer 12 and layers 12 and 14 are etched in a solution of HCL for a sufficient time to produce an optically flat mirror surface 40, with a small separation B' between the end of the laser active region 18 and the mirror surface 40.
The semiconductor diode lasers described in U.S. Pat. No. 4,468,850, as well as all other known diode lasers now in use, are very small discrete, edge-emitting devices, with the laser mirrors formed by cleaving, i.e., by breaking the substrate along certain crystallographic directions or, in the case of U.S. Pat. No. 4,468,850, by etching along a crystallographic plane. A need exists for a surface-emitting diode laser, i.e., one which emits laser radiation in a direction normal to or perpendicular to the substrate surface. Such a device lends itself to a major advantage of batch processing in a large-area chip containing a great number of laser diodes forming a two dimensional array along with other monolithic circuitry, since substrate-cleaving is no longer necessary.
Attempts to form a surface-emitting diode laser have not been particularly successful. For example, Iga et al. "Lasing Characteristics of Improved GaInAsP/InP Surface Emitting Injection Lasers", Electronic Letters, 23rd June 1983, Vol. 19, No. 13, have fabricated short cavity lasers in which light is propagated perpendicular to the plane of the p-n junction and the semiconductor layers. This type of structure suffers from the serious disadvantage that it is difficult to get sufficient cavity path length in such a structure. Also, to realize adequate gain to sustain laser emission, nearly 100% reflective mirrors are required.
Itaya, et al., "New 1.5 um Wavelength GaInAsP/InP Distributed Feedback Laser", Electronic Letters, 11th Nov. 1982, Vol. 18, No. 23 describe a distributed feedback DFB laser of GaInAsP/InP with an etched tilted facet wherein the DFB corrugations are designed to reinforce light waves propagated perpendicular to the active layer and obtain a laser beam coupled out normal to the plane of the p-n junction. This device suffers from the problem that gratings are difficult to make and that the coupling efficiency is low.
Surface-emitting lasers may find particularly useful application as monolithic two-dimensional arrays, as optical interconnects for integrated circuitry and as sources for optical pumping of solid state lasers.
Additionally, the process of forming etched mirror surfaces, as described in U.S. Pat. No. 4,468,850 would be significantly improved if the mirror surface could be formed without the use of an etchant to finally form the mirror surface. It is generally difficult, if not impossible, to obtain the flat perpendicular surfaces required for diode laser mirrors using etchants because the etchant process is extremely sensitive to crystallographic orientation and crystallographic defects.
Consequently, a need also exists for a method of forming mirror surfaces on diode lasers wherein the formation of the final mirror surface, per se, is achieved without the use of etchants.